Manufacturing method of silicon carbide single crystals

ABSTRACT

When a SiC substrate is heated up to around 1800° C., sublimation of SiC occurs from the SiC substrate. Moreover, temperature of the front surface of the SiC substrate is lower than that of the back surface of the SiC substrate. Therefore, sublimation gas sublimed from a back-surface vicinity of the substrate, where temperature is high, moves to a front-surface vicinity of the substrate, where temperature is low, through the hollow micro-pipe defect. Epitaxial growth proceeds on the front surface of the substrate while the sublimation gas is recrystallized at the front-surface vicinity of the substrate, so that the micro-pipe defect is occluded.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon Japanese Patent Application No.2000-377485 filed on Dec. 12, 2000, the contents of which areincorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to manufacturing method of asingle-crystal silicon carbide (SiC), especially to a method capable ofrestraining a micro-pipe as a crystal defect from being inherited.

2. Related Art

Heretofore, SiC single crystals are generally produced by sublimationmethod, however, hollow penetrating holes called micro-pipe defects(hollow penetrating defects) are formed at a degree of 100 to 1000pieces/cm².

In a case where a power device or a high frequency device is formed, anepitaxial film, which becomes regions to form devices, is formed so asto have a structure suitable for the devices using these single crystalsas a substrate. When the defects exist in the substrate, the defects areinherited into the epitaxial film which is grown on the substrate, sothat defects which are in the same number of the micro-pipe defects areformed in the epitaxial film. Further, when the devices are formed inthis epitaxial film with these defects, the fact is reported that leakcurrent of the devices increases while backward withstand voltagedecreases. Therefore, it is very important to reduce the defects inproducing the devices.

As a method for reducing the micro-pipe defects in the epitaxial film inwhich devices are formed, recently, methods for eliminating themicro-pipe defects in the SiC single crystals as the substrate has beenproposed. The methods are disclosed in U.S. Pat. No. 5,679,153,JP-A-10-324600, JP-A-2000-44398, and “Study on dislocations of 4H-SiCthick layer grown by CVD ” (The Lecture of the 47th Japan Society ofApplied Physics Related Association, Abstracts of the lecture, separatevolume 1, page 407, No. 29P-YF-6, Kamata et al., March, 2000, CentralResearch Institute of Electric Power Industry).

According to the method in U.S. Pat. No. 5,679,153, when crystals aregrown by liquid crystal epitaxy technique using melted SiC in silicon,an epitaxial film in which micro-pipe defects are reduced is grown on aseed substrate having micro-pipes.

Next, according to the method in JP-A-10-324600, formation of apolycrystalline film of a β(cubic)-SiC or α(hexagonal)-SiC on a surfaceof an α-SiC single crystal substrate (seed crystal) by thermal chemicalvapor deposition (CVD) and thermal treatment of the composite bodyresulting from the formation are repeated a plurality of times so that aplurality of α-SiC or β-SiC polycrystalline films are oriented (the kindof solid phase epitaxial growth) in the same direction of the crystalaxis of the α-SiC single crystal substrate (seed crystal). Thus, SiCsingle crystals are formed so as to have few micro-pipe defects.

On the other hand, according to JP-A-2000-44398, after a coatingmaterial is coated on a single crystal substrate having micro-pipes,thermal treatment is conducted to occlude the micro-pipe defects in theSiC substrate that exist in the SiC substrate, so that a crystal inwhich at least a part of the micro-pipe defects are occluded isobtained.

Further, according to the Abstracts in The Lecture of the 47th JapanSociety of Applied Physics Related Association, the fact is reportedthat an epitaxial film is formed on a substrate in a thickness of 65 μmat a rate of 16 μm/h, so that micro-pipes are occluded.

According to the above-described first method, the epitaxial film shouldbe grown to a thickness of about 20 to 75 μm or more by the liquid phaseepitaxy method, to obtain a region where the micro-pipes are eliminated.Moreover, an epitaxial film on which devices are formed is formed on theepitaxial film by liquid phase epitaxy by a CVD method, so that a numberof manufacturing processes increase.

According to the above-mentioned second method, SiC composite isobtained so as to include crystal boundaries therein since thepolycrystalline film is formed on the single crystal substrate. When thecomposite is subjected to the thermal treatment to cause the solid phaseepitaxy on the seed crystal, there is possibility that crystal defectsdue to internal stress at the crystal boundaries in the polycrystallinefilm are introduced. These defects become sources of traps, andtherefore there is a problem that this substrate is not suitable for asubstrate to form devices. Moreover, the formation of the film, thethermal treatment, and a surface flattening should be repeated severaltimes to grow a substrate having a practical thickness. Thus, processesincrease so that manufacturing cost becomes high.

According to the above-mentioned third method, at least the coveringprocess with the coating material, the thermal treatment, and a surfaceflattening process that includes a removing process of the coatingmaterial are necessary, so that the manufacturing process increases.

According to the above-mentioned fourth method, although the micro-pipesare occluded by thickening the epitaxial film, a thickness of theepitaxial film to form devices on the substrate is about 20 to 30 μm atmost. Therefore, there is a need that the micro-pipes are occluded evenif the epitaxial film is thin. Besides, the growth rate only about 16μm/h. It takes many hours, i.e., 4 hours or more to occlude themicro-pipes. That is, this method is not suitable for a commercial useas a method for forming an epitaxial film for devices or bulk.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedproblem, and an object thereof is to provide a technique for providingmore practical occluding method of a micro-pipe in a silicon carbidesubstrate that has the micro-pipe.

According to a first aspect of the present invention, when an epitaxialfilm is formed on a silicon carbide substrate having a micro-pipe,temperature difference is applied between a front surface of thesubstrate and a back surface of the substrate that is disposed oppositeto the front surface so that the front surface is at a low temperature.

By lowering the temperature at the front surface of the silicon carbidesubstrate as compared to the back surface, sublimation gas of SiC isgenerated at the vicinity of the back surface where the temperature ishigh (the micro-pipe or the back surface). The sublimated gas flows to aside of the front surface through the micro-pipe, and thenrecrystallized at the vicinity of the front surface where thetemperature is low. At that time, the gas is recrystallized at an insideof the micro-pipe, so that an inner diameter of the micro-pipe becomessmall, and finally, the micro-pipe can be occluded.

Incidentally, as described in a second aspect of the present invention,by setting a temperature of the substrate at 1650° C. or more,sublimation is apt to occur from the substrate, and occlusion of themicro-pipe is stimulated.

Moreover, as described in a third aspect of the present invention, bysetting the temperature of the substrate at 1750° C. or more,sublimation is stimulated from the substrate, so that the micro-pipe isoccluded easily. However, in a case where the temperature exceeds 1900°C., the sublimation is stimulated so that the sublimation and arecrystallization are balanced so as to restrain the occlusion of themicro-pipe. Therefore, the temperature of the silicon carbide substrateis preferably set to 1900° C. at most.

Moreover, as described in a fourth aspect of the present invention,since hydrogen gas or helium gas has a high heat-transmittingcharacteristic so as to effectively lower the temperature at the frontsurface of the silicon carbide substrate where the gas is supplied, sothat the temperature difference between the front surface and the backsurface of the substrate is sufficiently generated. Thus, thesublimation gas from the vicinity of the back surface of the siliconcarbide substrate is transferred to the front surface.

Moreover, as described in a fifth aspect of the present invention, bysetting a flow rate of the gas at 1 m/sec or more, the temperature atthe front surface of the silicon carbide substrate where the gas issupplied is effectively lowered so that the temperature differencebetween the front surface and the back surface of the substrate issufficiently generated. Thus, the sublimation gas from the vicinity ofthe back surface of the silicon carbide substrate is transferred to thefront surface.

Moreover, as described in a sixth or seventh aspect of the presentinvention, by setting the temperature difference between the frontsurface and the back surface of the silicon carbide substrate at 0.5,preferably, 5° C. or more, it can be encouraged that the gas sublimed ata side of the back surface is transferred, and recrystallized at thevicinity of the front surface. Thus, the micro-pipe can be occludedeasily.

Moreover, as described in a eighth aspect of the present invention, bysetting a growth rate of the epitaxial film at 20 μm/h or more, a growthrate toward a lateral direction of a silicon carbide film on themicro-pipe can be enhanced, so that the occlusion of the micro-pipe canbe shortened.

Moreover, as described in a ninth aspect of the present invention, whena thickness of the silicon carbide substrate is at 300 μm or more, thetemperature at the front surface of the substrate where the gas issupplied is effectively lowered, whereby the temperature differencebetween the front surface and the back surface of the substrate issufficiently generated. Thus, it is encouraged that the sublimation gasfrom the back surface is transferred to the front surface.

Moreover, as described in a tenth aspect of the present invention, bysetting flow direction of a gas containing carbon and a gas containingsilicon in approximately perpendicular to a front surface of thesubstrate that exposes an opening of said micro-pipe, a gas sublimedthrough the micro-pipe from the back surface is prevented from flowingout to the front surface. Therefore, it is encouraged that the sublimedgas is recrystallized at the vicinity of the opening. Thus, themicro-pipe can be occluded easily.

Moreover, as described in an eleventh aspect of the present invention,by setting a temperature of the substrate at 1650° C. or more,sublimation is apt to occur from the substrate, so that the micro-pipecan be easily occluded.

Moreover, as described in a twelfth aspect of the present invention, bypreferably setting the temperature of the substrate at 1750 to 1900° C.,sublimation is encouraged from the substrate, so that the micro-pipe isoccluded easily. The reason why the temperature is set at 1900° C. orless is that the sublimation is more encouraged as compared to therecrystallization at over 1900° C., and therefore heating up over 1900°C. is not preferable.

Furthermore, as described in a thirteenth aspect of the presentinvention, the micro-pipe penetrates the substrate from the frontsurface to the back surface, and the substrate is held so as to closelycontact a contacting member at the back surface thereof, so that thesublimed gas from the micro-pipe at the vicinity of the back surface isapt to move to the front surface, whereby the sublimed gas is encouragedto be recrystallized at the vicinity of the front surface. Thus, themicro-pipe can be easily occluded.

Moreover, as described in a fourteenth aspect of the present invention,the substrate is held so that pressure of an atmosphere contacting theback surface is high as compared to that of an atmosphere contacting thefront surface. As a result, the sublimed gas from the micro-pipe at thevicinity of the back surface is apt to move to the front surface,whereby the sublimed gas is encouraged to be recrystallized at thevicinity of the front surface. Thus, the micro-pipe can be easilyoccluded.

Moreover, as described in a fifteenth aspect of the present invention,by reducing the pressure in the epitaxial growth, the pressure at thevicinity of the back surface of the substrate is lowered through themicro-pipe, so that the sublimation of silicon carbide is encouraged.Thus, the micro-pipe can be easily occluded.

Moreover, as described in a sixteenth aspect of the present invention,when an opening of the micro-pipe is enlarged in the silicon carbidesubstrate, a plurality of steps are formed at the opening. Since thesteps are cores, a lateral growth of a silicon carbide film progresses,and therefore the micro-pipe can be easily occluded.

Moreover, as described in a seventeenth aspect of the present invention,by heating up the SiC substrate to 1650° C. or more in hydrogen, thefront surface of the silicon carbide substrate is etched. Specifically,an etching in the vicinity of defects is encouraged, so that the openingof the micro-pipe can be enlarged. Successively, by supplying a gascontaining carbon and a gas containing silicon, the epitaxial film canbe grown by epitaxial growth.

Moreover, as described in an eighteenth aspect of the present invention,by supplying a gas containing chlorine, the front surface of the siliconcarbide substrate is etched. Specifically, an etching in the vicinity ofdefects is encouraged, so that the opening of the micro-pipe can beenlarged. Successively, by supplying a gas containing carbon and a gascontaining silicon, the epitaxial film can be grown by epitaxial growth.

Moreover, as described in a nineteenth aspect of the present invention,by etching the silicon carbide substrate using KOH, the etching in thevicinity of the defects is encouraged, so that the opening of themicro-pipe can be enlarged.

Moreover, as described in a twentieth aspect of the present invention,when an enlarged diameter in the opening of the micro-pipe has a size oftwice or more as large as that before enlarged, the gas can be suppliedsufficiently, so that the growth at the opening is encouraged. Themicro-pipe can be easily occluded since a plurality of steps can beformed at the opening, and a growth of a silicon carbide film in alateral direction progresses while the steps serve as cores.

Incidentally, as described in a twenty-first aspect of the presentinvention, the silicon carbide substrate, on which the silicon carbidefilm is formed by the epitaxial growth in a chamber, serves as a seedcrystal, and a sublimation gas sublimed from a source material isgenerated in the chamber. Then, the sublimation gas is sublimed on theseed crystal.

Moreover, as described in a twenty-second aspect of the presentinvention, by forming the epitaxial film on a silicon carbide substratehaving a micro-pipe with an opening whose diameter increases as beingclose to a front surface of said substrate, a substrate can be obtained,in which the silicon carbide epitaxial film is not opened on themicro-pipe. Therefore a high quality silicon carbide single crystalsubstrate which has less micro-pipe is produced. Besides, the micro-pipeis occluded (or terminated) in this silicon carbide substrate, which canbe an advantage for producing devices since a thickness of the epitaxialfilm is usually considered in producing the devices without consideringa location of the micro-pipe.

Incidentally, as described in a twenty-third aspect of the presentinvention, a diameter of the opening, which is enlarged at a surface ofan opening of the substrate, is preferably twice or more as large asthat of the opening at a bottom of the opening of the substrate.

Moreover, as described in a twenty-fourth aspect of the presentinvention, when the micro pipe is occluded (or terminated) at aconductive region disposed between the silicon carbide substrate bodyand the epitaxial film, and the devices are formed in this substrate, ina case where a voltage is applied so as to expand a depletion layer, thedepletion layer that is expanded from the epitaxial layer is restrainedfrom being expanded by the conductive region so that the depletion layeris prevented from reaching the micro-pipe. Therefore, electric fieldconcentration at the micro-pipe, which is caused by the phenomenon inwhich the depletion layer reaches the micro-pipe, is suppressed so thata breakdown due to the micro-pipe can be prevented.

Incidentally, the conductive region is regarded as a region where animpurity concentration is high in comparison with a predeterminedepitaxial film.

Moreover, as described in a twenty-fifth aspect of the presentinvention, the conductive region may be a conductive substrate, or asdescribed in a twenty-sixth aspect of the present invention, theconductive region may exist in the epitaxial film.

Otherwise, as described in a twenty-seventh aspect of the presentinvention, the conductive region is a low resistivity epitaxial film. Inthis case, the depletion layer expanding from a high resistivityepitaxial film formed on the low resistivity epitaxial film isrestrained from expanding by the low resistivity epitaxial film. As aresult, the breakdown due to the micro-pipe is prevented.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description made withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a CVD apparatus for growing a SiCepitaxial film in the first embodiment;

FIGS. 2A to 2C are cross sectional views showing epitaxial growthprocess in the first embodiment, respectively;

FIGS. 3A and 3B are cross sectional views showing epitaxial growthprocess in the first embodiment, respectively;

FIG. 4 is a schematic view of a CVD apparatus for growing a SiCepitaxial film in the second embodiment;

FIGS. 5A and 5B are cross sectional views showing epitaxial growthprocess in the second embodiment, respectively;

FIGS. 6A and 6B are cross sectional views showing epitaxial growthprocess in the second embodiment, respectively;

FIGS. 7A and 7B are cross sectional views showing epitaxial growthprocess in the third embodiment, respectively;

FIGS. 8A and 8B are cross sectional views showing epitaxial growthprocess in the third embodiment, respectively;

FIG. 9A is a cross sectional view of the SiC epitaxial growth substrate;

FIG. 9B is a cross sectional view of the SiC epitaxial growth substratein the other embodiment, and

FIGS. 10A and 10B are cross sectional views showing modifications of theCVD apparatus in the second embodiment, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment adopting the present invention will beexplained. The present embodiment adopts the present invention to amethod for producing an epitaxial film for forming a device such as afield effect transistor (MOSFET or the like), a junction field effecttransistor (JFET), or a Schottky barrier diode on a silicon carbidesingle crystal substrate (SiC single crystal substrate) which isproduced by, for example, sublimation.

First Embodiment

FIG. 1 shows a schematic view of a (Chemical Vapor Deposition) (CVD)apparatus for forming an epitaxial film on a SiC single crystalsubstrate 10. The SiC single crystal substrate 10 is disposed in asusceptor 30 composed of carbon and having a cylinder shape. Thesusceptor 30 is surrounded by a heat insulator 31 so as to prevent heatradiated from the heated-up susceptor from leaking out to the outside.Moreover, the susceptor 30 and the heat insulator 31 are surrounded by areactor 32 which is composed of quartz. A coil 33 is disposed at aperiphery of that so that the susceptor 30 is heated up by highfrequency induction heating.

Hydrogen gas, SiH₄ gas and C₃H₈ gas are supplied through respectiveconduits, and are mixed up with each other just before a gas introducingconduit 34, and then supplied into the reactor 32 from the gasintroduction conduit 34. Exhaust is conducted by a rotary pump, which isnot shown, through a gas exhausting conduit 35.

Hereinafter, an example in which epitaxial growth is performed in thisapparatus will be explained using FIGS. 2A-3B.

First, a 6H-SiC single crystal substrate 10 is provided as the SiCsingle crystal substrate, which has an off-surface inclined at 3.5degrees from a (0 0 0 1) surface, and has a thickness of 700 μm (FIG.2A). In this case, since a micro-pipe defect 11 extends in a directionof <0 0 0 1> axis, the micro-pipe defect 11 is inclined with respect toa front surface of the SiC substrate 10. This substrate 10 is disposedon the susceptor 30 and inserted into the reactor 32 of the CVDapparatus.

Further, the substrate 10 is heated up to 1800° C. while hydrogen isflowed as a carrier gas 13 a at a reduced pressure of, for example, 200Torr (FIG. 2B). At that time, a back surface of the substrate 10contacts the susceptor while the front surface of the substrate 10 issubjected to hydrogen. Besides, flow rate of hydrogen is faster, such as1 m/sec or more in the reduced pressure of 200 Torr. Therefore, heat isapt to be taken away from the front surface of the substrate 10, so thattemperature at the front surface is kept low.

Incidentally, temperature of the substrate 10 is regarded as temperatureof the susceptor 30 which is measured by a pyrometer.

After the temperature of the substrate 10 reaches 1800° C., a mix gas,in which the SiH₄ gas and C₃H₈ gas as source gases are added to thehydrogen gas, is introduced, so that a SiC epitaxial film 14 is grown onthe front surface of the substrate 10. Incidentally, a flow of thehydrogen gas is at 10 liters/min.

At high temperature of 1800° C., sublimation of SiC occurs from the SiCsubstrate 10 so that sublimation gas 10 a such as Si₂C, SiC₂ and thelike is generated. Moreover, temperature of the front surface of thesubstrate is lower than that of the back surface of the substrate.Therefore, sublimation gas sublimed from a back-surface vicinity 12 b ofthe substrate, where temperature is high, moves to a front-surfacevicinity 12 a of the substrate, where temperature is low, through themicro-pipe defect 11 which is hollow.

Meanwhile, at the front surface of the substrate 10, the source gasesflow with the hydrogen. By thermal decomposition of the source gases, aplurality of Si atoms and a plurality of C atoms exist at the frontsurface of the substrate 10 to form the epitaxial film, so that thesublimation gas sublimed from the vicinity of the back surface of thesubstrate 10 is not apt to diffuse from the front surface to the outsideof the substrate 10, whereby the sublimation gas is recrystallized on aninner wall of the micro-pipe defect that is disposed at the vicinity ofthe front surface of the substrate 10 (FIG. 2C).

Moreover, an epitaxial film 14 grows in a direction of an a-axisperpendicular to a thickness direction of the substrate 10(perpendicular to a <0 0 0 1> axis), and therefore, grows on arecrystallized region on the micro-pipe defect 11 to encourage anocclusion of the micro-pipe defect 11.

As time passes, crystals increases which are recrystallized at the innerwall of the micro-pipe defect 11, at last, the micro-pipe defect 11 isoccluded while the epitaxial film grows on that, so that the micro-pipedoes not extend into the epitaxial film 14 (FIGS. 3A and 3B).

As described above, in this embodiment, the front surface is cooled downas compared with the back surface of the substrate 10 by flowingreaction gases for epitaxial growth at a predetermined rate, and thesublimation gas sublimed at the vicinity of the back surface of thesubstrate 10 is recrystallized at a front surface side. By utilizingthis phenomenon, the micro-pipe defect 11 can be occluded.

The SiC substrate 10 is thick, such as 700 μm, it is easy to cause atemperature difference between the front surface and the back surface ofthe substrate 10.

Moreover, since the flow rate is fast, growth rate is high, such as 50μm/h. Even if a thickness of the epitaxial film is 10 μm, the micro-pipe11 can be occluded at the front surface of the SiC substrate 10 withoutbeing absorbed into the epitaxial film.

Incidentally, a direction of the gas flow and a disposed orientation ofthe substrate are not limited to the case shown in FIG 1. The frontsurface of the substrate may be oriented to a lower side. Moreover, thegases may flow in an up-and-down direction and the front surface of thesubstrate is disposed in parallel with the flow.

Second Embodiment

FIG. 4 shows a schematic view of a CVD (Chemical Vapor Deposition)apparatus for forming an epitaxial film on a SiC single crystalsubstrate 10. The SiC single crystal substrate 10 is disposed in asusceptor 30 composed of carbon and having a cylinder shape. In thisapparatus, the SiC substrate 10 is disposed so that a front surfacethereof faces a lower side. The SiC substrate 10 is adhered to apedestal 36 composed of carbon to be fixed. Incidentally, although notshown in the figure, the pedestal 36 is fixed to the susceptor 30. Thepedestal 36 serves as a member for heating the substrate 10 andequalizing heat as well as for fixing of the SiC substrate 10.

The susceptor 30 is surrounded by a heat insulator 31 so as to preventheat radiated from the heated-up susceptor from leaking out to theoutside. Moreover, all of them are surrounded by a reactor 32 which iscomposed of quartz. A coil 33 is disposed at a periphery of that so thatthe susceptor 30 is heated up by high frequency induction heating.

Hydrogen gas, SiH₄ gas and C₃H₈ gas are supplied through respectiveconduits, and are mixed up with each other just before a gas introducingconduit 34, and then supplied into the reactor 32 from the gasintroduction conduit 34. Exhaust is conducted by a rotary pump which isnot shown through a gas exhausting conduit 35.

Hereinafter, an example in which epitaxial growth is performed in thisapparatus will be explained using FIGS. 5A-6B. Conditions for growth arethe same as the first embodiment.

First, a 4H-SiC single crystal substrate is provided, which has anoff-surface inclined at 8 degrees from a (0 0 0 1) surface, and has athickness of 300 μm (FIG. 5A).

Further, the substrate 10 is heated up to 1800° C. while hydrogen isflowed as a carrier gas 13 a at a reduced pressure of, for example, 200Torr (FIG. 5B). At that time, a back surface of the substrate 10contacts the susceptor while the front surface of the substrate issubjected to hydrogen. Besides, flow rate of hydrogen is faster, such as1 m/sec or more in the reduced pressure of 200 Torr. Therefore, heat isapt to be taken away from the front surface of the substrate 10, so thattemperature at the front surface is kept low.

After the temperature of the substrate 10 reaches 1800° C., a mix gas inwhich the SiH₄ gas and C₃H₃ gas as source gases are added to thehydrogen gas is flowed, so that a SiC epitaxial film 14 is grown on thefront surface of the substrate 10. Incidentally, a flow of the hydrogengas is at 10 liters/min.

At high temperature of 1800° C., sublimation of SiC occurs from the SiCsubstrate 10. Moreover, temperature of the front surface of thesubstrate is lower than that of the back surface of the substrate.Therefore, sublimation gas sublimed from a back-surface vicinity 12 b ofthe substrate, where temperature is high, moves to a front-surfacevicinity 12 a of the substrate, where temperature is low, through themicro-pipe defect 11 which is hollow.

Meanwhile, at the front surface of the substrate 10, the source gasesflow with the hydrogen toward the substrate, so that the sublimation gassublimed from the vicinity of the back surface of the substrate 10cannot exit from the front surface to the outside of the substrate 10.Therefore, recrystallization on an inner wall of the micro-pipe defect11 at the vicinity of the front surface is possible. As time passes, thenumber of crystals that are recrystallized at the inner wall of themicro-pipe defect 11 increases. Finally, the micro-pipe defect 11 isoccluded while the epitaxial film grows on that the micro-pipe defect,so that the micro-pipe does not extend in the epitaxial film 14 (FIGS.6A and 6B).

Incidentally, a direction of the gas flow and a disposed orientation ofthe substrate are not limited to the case shown in FIG. 4. The gases mayflow from an upper side and the front surface of the substrate may beoriented the upper side.

Third Embodiment

In this embodiment, similarly to the above-mentioned second embodiment,another example in which epitaxial growth is performed in the apparatusshown in FIG. 4 will be explained using FIGS. 7A-8B.

First, a 6H-SiC single crystal substrate is provided, which has anoff-surface inclined at 3.5 degrees from a (0 0 0 1) surface. Thesubstrate is disposed in a susceptor 30, and inserted in the reactor ofthe CVD apparatus (FIG. 7A).

Further, the substrate 10 is heated up to 1800° C. while hydrogen isflowed as a carrier gas 13 a at a reduced pressure of, for example, 200Torr (FIG. 5B), and the substrate 10 is kept at that condition for 10minutes. At that time, defect is selectively etched by exposing a frontsurface of the substrate to hydrogen at high temperature ofapproximately 1800° C., so that an opening 12 c of a micro-pipe 11 thatis located at the front surface as shown in FIG. 7B is enlarged incomparison with a diameter of the micro-pipe (a bottom of the opening 12c). As a result, the micro-pipe has a pipe portion 12 d and the openingportion 12 c therein. Preferably, a diameter of the opening 12 c at atop thereof is twice or more as large as that at a bottom thereof wherethe pipe portion 12 d is connected.

After that, SiH₄ gas and C₃H₈ gas as source gases are introduced to forma SiC epitaxial film 14 on the front surface of the substrate. At thattime, the epitaxial film 14 grows while a growth in a lateral directionprogresses. The growth is faster than that on the front surface, due toa synergistic effect in which a plurality of steps are formed at anenlarged opening as cores for growth, and a surface of the openingapproximates the a-surface from a face orientation of the front surface.As a result, a thickness of the epitaxial film becomes thicker, andfinally the micro-pipe 11 is occluded, and the epitaxial film grows onthe micro-pipe 11. Therefore, the micro-pipe defect 11 is not absorbedinto the epitaxial film (FIGS. 8A and 8B).

Further, in this embodiment, similar to the second embodiment, themicro-pipe defect 11 is occluded by a sublimation gas of SiC.

Incidentally, as a method for forming the opening at the surface of themicro-pipe defect 11, a gas containing chlorine instead of hydrogen maybe introduced to obtain a similar effect. In a case where chlorine isemployed, it is unnecessary to heat up the substrate. In the case wherehydrogen is employed, it is preferable that the substrate is heated upto 1650° C. or more. This is because etching effect by using hydrogen isnot apt to appear at low temperatures, and therefore it takes long timeto enlarge the opening at the surface of the micro-pipe defect 11.

Also, in a case where a SiC single crystal substrate is etched in a KOHsolution at approximately 500° C., the substrate is disposed in the CVDapparatus, and SiC epitaxial growth is conducted, the similar effect isobtained.

Other Embodiment

Although the embodiments of the present invention are described above,the occlusion of the micro-pipe defect is not necessarily achieved inthe SiC substrate 10 in order to produce devices. In this embodiment, asanother example shown in FIGS. 9A and 9B, an occluded location ofmicro-pipe defect will be explained in view of a relation between awithstand voltage of the devices and the micro-pipe defect.

When a depletion layer reaches the micro-pipe defect, a breakdownoccurs. In this case, the breakdown voltage is lower than that expected.Therefore, it is preferable that the depletion layer does not reach themicro-pipe defect.

FIG. 9A shows a n⁺-type low resistivity substrate 20 in which impuritiesare introduced at high concentration (for example, 10¹⁹ to 10²⁰/cm³) andon which a high resistivity, n⁻-type epitaxial film 14 is formed bymethod described above.

FIG. 9A shows the low resistivity substrate 20 on the n⁻-type epitaxialfilm 14 having a low impurity concentration. In this case, when a p-typeregion is formed in or on the n⁻-type epitaxial film 14 to serve as adevice, and when a reverse bias is applied to a p-n junction formed inthe substrate, there may be a case where the depletion layer expands topenetrate the n⁺-type epitaxial film 14 and reach the SiC substrate 20.Since an impurity concentration of the SiC substrate 20 is high, thedepletion layer hardly expands in the SiC substrate 20.

Therefore, when the micro-pipe defect 11 is occluded in the SiCsubstrate 20, the depletion layer does not reach the micro-pipe defect11, and therefore the breakdown due to the micro-pipe defect 11 isprevented from occurring.

Moreover, FIG. 9B shows the n⁺-type low resistivity substrate 20 onwhich a low resistivity n⁺-type epitaxial film 21 similar to thesubstrate 20 is formed, and a high resistivity, n⁻-type epitaxial film22 is formed on the low resistivity n+-type epitaxial film 21.

A method in which the n⁺-type epitaxial film 21 is formed at 1750° C. orless may be utilized to form the above structure. Also in this case,similar to the case shown in FIG. 9A, it is preferable that themicro-pipe defect 11 is occluded in the low resistivity region since themicro-pipe defect 11 may not influence the withstand voltage.

Namely, preferably, the micro-pipe defect should be occluded in aconductive low resistivity region. Incidentally, the word “conductive”means a low resistivity region to which impurities are introduced tosuch a degree that this region can serve as a conductor.

FIGS. 10A and 10B show a modification of the structure, shown in FIG. 4,to hold the SiC substrate 10 in the CVD apparatus.

As shown in FIG. 10A, a substrate holder 37 protrudes from a side faceof the susceptor 30 to hold SiC substrate 10. A heat equalizer 38composed of carbon is disposed on the back surface of the SiC substrate10 so as to closely contact the back surface of the SiC substrate 10.All portions of the SiC substrate 10 are equally heated up to thetemperature of 1750° C. or less by the heat equalizer 38.

As such, by making the heat equalizer 38 contact the back surface of theSiC substrate 10, even if the micro-pipe defect penetrates the SiCsubstrate 10, the micro-pipe defect is occluded at the front surface ofthe SiC substrate 10. The sublimation gas sublimed from the vicinity ofthe back surface of the SiC substrate 10 is apt to move to the frontsurface of the SiC substrate 10 by epitaxial growth. As a result, theocclusion of the micro-pipe defect occurs.

Moreover, as shown in FIG. 10B, the heat equalizer 38 does not contactthe SiC substrate 10 with a space interposed therebetween, but contactsthe substrate holder 37. The substrate holder 37 holds an entireperiphery of the SiC substrate 10 to form a closed space between theheat equalizer 38 and the SiC substrate 10. Therefore, a pressure due tothe closed space is applied to the back surface of the SiC substrate 10while a decompressed atmosphere exists on the front surface of the SiCsubstrate 10 to such a degree of, for example, 200 Torr which is lowpressure as compared to the back surface.

Incidentally, the heat equalizer 38 may not contact the substrate holder37, and a narrowed space may be formed between the heat equalizer 38 andthe SiC substrate 10 that is only about several mm or less.

Therefore, the sublimation gas sublimed from the vicinity of the backsurface of the SiC substrate 10 is apt to move to the front surface ofthe SiC substrate 10, so that the occlusion of the micro-pipe defect isadvanced to the front surface of the SiC substrate 10.

As described above, when the micro-pipe defect is occluded using thetemperature difference between the front surface and the back surface ofthe SiC substrate 10, a pressure difference between the front surfaceand the back surface of the substrate 10 is generated. In other words,the pressure applied to the front surface is lowered in comparison withthe back surface to permit the occlusion of the micro-pipe defect.

Incidentally, although not shown in the figure, the substrate holder 37has an opening at a portion other than the region to contact the SiCsubstrate 10 so that the gas flowing into the susceptor 30 from thelower side is exhausted from the upper side of the susceptor 30.

Other methods will now be explained.

Preferably, the temperature of the SiC substrate is at 1650° C. or moresince sublimation is apt to occur from the SiC substrate at thistemperature condition. For further advancing the sublimation, thetemperature is at 1750° C. or more, preferably 1800° C. or more.

Incidentally, when the temperature of the SiC substrate exceeds 1900°C., sublimation is preferred rather than recrystallization, so thatthere is a possibility that the micro-pipe defect cannot be occluded.Therefore, the temperature is preferably set at 1900° C. or less.

However the temperature can be at 1900° C. or more, for example,approximately at 2250° C. which is a temperature of a seed crystalsubstrate in the sublimation method when the conditions of the growthrate, the atmosphere in the growth and the like can be suitably set.

Moreover, in terms of cooling the front surface of the SiC substrate,the flow rate of the carrier gas or the gas for the epitaxial growth ispreferably set at 1 m/sec or more.

Moreover, the temperature difference between the front surface and theback surface of the SiC substrate is set at 0.5° C. or more, preferablyset at 5° C. or more. As such, the sublimation gas from the vicinity ofthe back surface of the SiC substrate is transferred to the frontsurface and is apt to be recrystallized through the micro-pipe defect.

Moreover, when the thickness of the SiC substrate is at 300 μm or more,the temperature difference between the front surface and the backsurface is apt to be generated.

Moreover, by setting the growth rate of the epitaxial film to be formedon the front surface of the SiC substrate at 20 μm/h or more, preferably30 μm/h or more, the growth rate toward the lateral direction (a-facegrowth) approximately perpendicular to the thickness direction of thesubstrate can be enhanced, so that the micro-pipe defect can beprevented from being absorbed into the epitaxial film.

Incidentally, the present invention is not limited to a substrate for adevice, but may be employed in a bulk growth by utilizing a benefit thatthe growth rate of the epitaxial film is fast. In this case, SiC singlecrystals can be obtained in which micro-pipe defects are eliminated.Moreover, the SiC substrate, on which the epitaxial layer is grown bythe method as described above to occlude the micro-pipe defect, can beemployed as a seed crystal for the so-called sublimation method in whichSiC source powder or source gases are sublimed to be recrystallized onthe seed crystal so as to form a bulk SiC. In this case, sublimed gasesare recrystallized on the epitaxial film disposed in a chamber composedof, for example, carbon.

Moreover, although the case where SiH₄ gas and C₃H₈ gas serve as sourcegases is described, hydride such as Si₂H₆ gas, C₂H₄ gas or the like,chloride, sublimation gas of SiC, or Si vapor may be employed as thegas. Furthermore, the growth method is not limited to the CVD method,but can employ a vapor phase deposition method such as molecularepitaxial growth method, sublimation method or the like.

What is claimed is:
 1. A silicon carbide substrate comprising: a siliconcarbide single crystal body having a hollow micro-pipe having apipe-shaped portion, and an opening connected with the micro-pipe andhaving a diameter that increases closer to a front surface of saidsilicon carbide single crystal body, and a plurality of steps are formedon a wall surface of the opening of the silicon carbide single crystalbody, wherein the pipe-shaped portion is located at a side of a backsurface of the silicon carbide single crystal body, and the opening islocated at a side of the front surface of the silicon carbide singlecrystal body; and a silicon carbide epitaxial film formed on the frontsurface of the silicon carbide single crystal body so as to cover themicro-pipe, wherein the micro-pipe is not absorbed into the siliconcarbide epitaxial film.
 2. A silicon carbide substrate according toclaim 1, wherein: the diameter of the opening at a top thereof, is atleast twice as large as the diameter of the opening at a bottom thereofwhere the pipe-shaped portion is connected.
 3. A silicon carbonsubstrate comprising: a conductive silicon carbide substrate body havinga micro-pipe; and a silicon carbide epitaxial film formed on a frontsurface of the conductive silicon carbide substrate body, wherein thesilicon carbide epitaxial film covers the micro-pipe, and the micro-pipeis terminated within the conductive silicon carbide substrate body, andwherein the conductive silicon carbide substrate body has lowresistivity and the silicon carbide epitaxial film has high resistivity.4. A silicon carbide substrate comprising: a conductive silicon carbidesubstrate body having a micro-pipe; a low resistivity conductive siliconcarbide epitaxial film defining a conductive region and formed on afront surface of the conductive silicon carbide substrate body, whereinthe micro-pipe is terminated within the conductive region of the lowresistivity conductive silicon carbide epitaxial film; and a highresistivity conductive silicon carbide epitaxial film formed on a frontsurface of the low resistivity conductive silicon carbide epitaxialfilm.
 5. The silicon carbide substrate of claim 4, wherein the lowresistivity silicon carbide epitaxial film covers the micro-pipe, andthe micro-pipe is terminated within the low resistivity silicon carbideepitaxial film.